1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method for power control in a multi-layer base station during a handoff.
2. Background of the Related Art
In a mobile communication system such as a code division multiple access (CDMA) communication system, a handoff must be executed when a mobile station moves from one service region to another in order to receive a continuous communication service from a base station without interruption. Such handoffs may be classified into a soft handoff, a softer handoff, and a hard handoff, depending on a destination base station, a handoff time and the use of channel resources between a serving base station and the destination base station. Here, a serving base station means a base station which provides a service to a mobile station before a handoff and a destination base station means a base station which will provide service to the mobile station after a handoff.
Generally, a soft handoff occurs when a mobile station moves from one base station to another base station wherein the base stations have a common frequency and frame offset. In the soft handoff, a communication with a new base station begins before communication with a base station currently in service ends, thereby maintaining a continuous traffic path. A softer handoff occurs when a mobile station moves between sectors of a same base station having the same frequencies. A hard handoff occurs when a mobile station moves from one base station to another base station belonging to a different mobile switching center (MSC) wherein the base stations have traffic channels of different frequencies or traffic channels of different frame offsets. In the hard handoff, communication with a base station currently in service ends before communication with a new base station begins. A mobile communication system and the handoff operation will next be explained.
FIG. 1 shows a mobile communication system in the related art including mobile stations 100a˜100n, base stations 101a˜101n a base station controller 102, a base station controller manager 103, and a mobile switching center 104. The base stations 101a˜101n may use a same frequency while transmitting pilot signals of different frame offsets to the mobile stations 100a˜100n, and the mobile stations 100a˜100n receives these pilot signals to distinguish the base stations 101a˜101n. FIG. 2 illustrates a service region of a base station in FIG. 1.
Referring to FIG. 2, a base station A provides service to mobile stations in base station A region and a base station B provides service to mobile stations in base station B region. Each service region is divided into a plurality of sectors called a cell. Also, as a mobile station moves, the mobile station may pass through a handoff region in which service can be provided by both base stations A and B.
FIG. 3 is a chart showing the flow of signals during a handoff in a mobile communication system in the related art. Generally, a handoff is executed in a mobile communication system when a mobile station moves from one cell to another, and particularly, a soft handoff is executed when a mobile station moves from one base station to another base station controlled by a same base station controller. For purposes of explanation, assume base station A is a serving base station of a mobile station, and base station B is a destination base station of the mobile station.
Referring to FIG. 3, a mobile station first transmits a Pilot Strength Measuring Message (PSMM) to base station A if the mobile station determines that a pilot signal, which is not from base station A, has a strength higher than a predetermined threshold value T_ADD (S300). The base station controller then analyzes the PSMM from the mobile station and requests base station B to allocate a channel to execute a soft handoff, if the pilot signal is determined to be from base station B rather than base station A. Upon allocating a channel according to the request from the base station controller, base station B notifies the base station controller of the allocation. Thereafter, the base station controller transmits an extended handoff direction message to the mobile station through both base station A and base station B to set a new active group (S301). An active group includes the mobile station and base stations having communication channels established with the mobile station.
Accordingly, the mobile station enters a soft handoff state after adding base station B to the active group and transmitting a handoff complete message to both base station A and base station B. Thus, the mobile station can receive service by base station B without being cut off from communication (S302).
While taking into consideration the number of subscribers as well as the quality of communication, the CDMA mobile communication system performs a power control such that signals from different mobile stations are received at a base station with relatively same power level. Under the power control, a mobile station located far from a base station transmits signals at a relatively high power level and a mobile station located close to a base station transmits power at a relatively low power level. This power control is also performed during a handoff.
Particularly, a base station transmits a power control command to a mobile station to reduce the power if the base station receives a transmission signal power, which is higher than average, while a handoff is being executed. Similarly, the base station transmits a power control command to the mobile station to increase the power if the base station receives a transmission signal power, which is lower than average, while a handoff is being executed. Moreover, mobile stations located in a handoff region may receive a power control command from more than one base stations, depending on the characteristic of the handoff. As a result, a mobile station does not increase the power until a power control command to increase the power is received from all base stations involved in the handoff, i.e. the serving base station and the destination base stations.
However, in the power controlling method of the related art, a handoff cannot be smoothly executed in multi-layered cells such as a macrocell and a picocell, as shown in FIG. 4.
Referring to FIG. 4, assume that service in a macrocell is provided by base station A and service in picocells are provided by base stations B and C, respectively. In the case shown, base station B does not match the power levels of a forward link coverage and a reverse link coverage. As a result, a mobile station in the picocell region of base station B does not transmit signals at a maximum power at the forward link coverage. On the other hand, base station C matches the power levels of the forward link coverage and the reverse link coverage, and a mobile station in the picocell region of base station C transmits signals at a maximum power at the forward link coverage.
Under the above state, if a mobile station enters a handoff region between a picocell region of base station B and a macrocell region of base station A while moving from the picocell to the macrocell, base station B does not transmit a power control command to the mobile station to increase the signal power. As a result, even if the mobile station determines that a pilot signal from base station A has a strength higher than the predetermined threshold value T_ADD, the power of a PSMM signal from the mobile station (S500), as received by base station A, would be too low. Thus, an extended handoff direction message (S501) as well as a handoff complete message (S502) required in a normal handoff procedure cannot be transmitted, and a soft handoff cannot be executed as shown in FIG. 5.
Similarly, if a mobile station enters a handoff region between a picocell region of base station C and a macrocell region of base station A while moving from the macrocell to the picocell, base station A does not transmit a power control command to the mobile station to increase a signal power. Accordingly, the power of a PSMM signal from the mobile station, as received by base station C, would be too low, and an extended handoff direction message as well as a handoff complete message required in a normal handoff procedure cannot be transmitted. Thus, a soft handoff cannot be executed.
As described above, in a mobile communication system having multi-layered cells, a handoff may not be executed smoothly because a channel would be allocated to only one of the base station during a handoff. Therefore, a constant level of quality in the communication service to users cannot be maintained, limiting the system capacity.